Firstly, whole grains contain much more fibre than refined “white” grains, such as white rice or bread made with white flour.

Dr Lim Su Lin, chief dietitian at the National University Hospital, said that, for instance, a bowl of boiled, unpolished brown rice contains 2.7g of dietary fibre. A bowl of similarly-cooked white rice contains only 1.8g.

And a bowl of rolled oats, cooked with water, contains 3.38g of fibre, while a similar portion of cooked quinoa contains 5.2g of fibre.

Much of this fibre comes from two parts which form the outer parts of each individual grain, called the bran and germ.

The remaining endosperm, which we eat when we consume refined grains, consists mainly of starchy carbohydrates and proteins.

“Unfortunately, most of the fibre and nutrients which are present in the bran and germ are lost during the refining process,” Dr Lim said.

“Fibre causes food to stay in the stomach longer, causing a person to feel full so they eat less.”

This means that whole grains could help an overweight person on their weight loss journey.

In addition, she said, fibre in whole grains also helps to prevent constipation and acts as a food source for good bacteria in the stomach.

Apart from providing fibre, the bran and germ portions of whole grains also serve as an important source of nutrients, including magnesium, potassium and vitamin B.

Ms Wong Hui Xin, a dietitian at Mount Elizabeth Novena Hospital, said that, for instance, corn has one of the highest levels of phenolic acids compared with other wholegrains such as wheat or oats.

These acids are found in some plants and work as antioxidants by slowing down oxidation – a natural process when oxygen is metabolised – that is part of normal bodily function but also damages cells.

Dr Nitish Mishra, an endocrinologist at Raffles Hospital, said the outer layers that remain on whole grains make them more difficult for the body to break down.

“The grains don’t release carbohydrates easily, so it takes the body longer to digest them. If the grain breaks down too quickly, glucose spikes too fast and the body has to produce more insulin,” he added.

In the long term, the pancreas becomes less efficient at producing insulin when sugar spikes are too frequent. The end result can be diabetes, which affects more than 400,000 people in Singapore, and can lead to complications such as blindness, amputation and heart attacks.

However, Dr Iain Brownlee from Newcastle University, which has a School of Agriculture, Food and Rural Development campus in Singapore, warned that it is not as simple as merely switching from refined to whole grains.

It is difficult to sieve out the precise factors that make a person healthy, he said.

This means that it is difficult to know exactly what impact whole grains have on an individual’s health.

Dr Brownlee, who is director of operations for food and human nutrition at the university, said: “There is a tendency that those who have consumed high amounts of whole grains also tend to smoke less and eat more healthily.”

In 2010, he did a study on a group of people who did not frequently eat whole grains and put them on a wholegrain diet.

However, the team found that this had no impact on health markers like blood glucose or body weight over a four-month period.

This could be that they needed to change their diets rather than just switching to whole grains, in order to have a measurable effect on health outcomes, Dr Brownlee said.

He suggested that people should follow the Health Promotion Board’s dietary advice, which recommends a balanced diet of fruit and vegetables, whole grains and meat.

As for whole grains, the experts say there is no single “best” grain to eat. “For example, hulled barley contains higher fibre compared to rolled oats, but the latter is a better source of phosphorus, manganese and thiamin,” said Ms Christine Rubi-Cruz, a wellness and lifestyle manager at Cold Storage Singapore.

“The key is to incorporate as much variety in the diet as possible.”

A version of this article appeared in the print edition of The Straits Times on May 31, 2016, with the headline ‘Jury still out on impact of whole grains’.